Wrong size? Wrong style? Wrong brand? Wrong expectations? With binders, it isn't one size fits all! Sometimes you have to play around with everything to figure out what works best for you - don't be discouraged if your first binder isn't right. When i first started binding i probably exchanged binders for a different size/style/brand a dozen times! Most places are very understanding.
You also have to get used to putting them on, adjusting yourself inside of them, and wearing them. Sometimes you have to move things around to get the desired look. You also have to accept that you're never going to be perfectly flat while binding - unless you're tiny, there is going to be a lump in that general area. Gotta learn how to position it, gotta learn how to dress and layer over it, and that's really the key to binding IMO. Learning how to work with what's left after you've found the right size binder!
TBH the GC2B binders have always felt loose, at least in comparison to the Underworks binders. I have the half top and the full length versions, the half top binds better than the full length but i still feel like i'm... is "bouncing" the right term? It feels loose. That doesn't mean it isn't binding, though - with layers on overtop i'm comfortable enough with both binders to wear them to work (they're more comfortable for my job anyway since i stand all day, lift, etc.) I pass well enough.
In comparison the Underworks binder almost always feels restrictive. No "bouncing" in that one unless i wear it every day for a week - then i pop it in the laundry and it goes back to "normal". I feel like it does a better job binding, but it's not as comfortable - in fact the new one i just got to replace the one that's falling apart is very rough and scratchy. It's left marks, which i never remember being a problem before (maybe they changed the fabric?!?)
So you may just have to play around with everything to figure out what works best for you. I also want to point out that if you're looking down at yourself, or at yourself in the mirror without a top on, you're probably not going to get the same effect as someone looking at you with a shirt (or layers) on is going to get. I always see "chest" when i look at myself, even if no one else does. It's a mental thing. I've learned not to look at myself unless i have my shirt on, or i'll always be disappointed.